This is exactly why I left the consulting biz too. I had a web development agency with a bunch of friends & ex-colleagues that was about to make $1M in profits after 3 years. That number would be much higher, had it not been for clients (mostly startups and small businesses) that weren't able to pay (so they claimed) or went out of business. This is despite collecting a deposit of 50% of the estimated costs up-front.
Before we broke $1M, I saw a great opportunity to develop a product, so I shifted my focus and have been doing that ever since.
Consulting is good for getting (relative) immediate cash up-front, while products can take a long time to begin earning that same amount of cash. However, if done well, products can scale and earn much more revenue in the long run.
Nathan is definitely taking the right approach for an individual entrepreneur. He's diversifying his income (consulting, apps, books) while presumably moving his focus away from consulting as his income from apps and books increases.
Good luck man!